FIGS. 12 and 13 show a conventional water pump and an impeller thereof used in a cooling system of an engine, respectively. The water pump is mounted on a wall 2 at the side of an engine block having a suction opening 1. The water pump includes a pump housing 4 having a volute casing 3, and a rotary shaft 5 rotatably supported in a cylindrical projection of the pump housing 4. An impeller 10 made of steel plate having a boss 10a and blades 10b is firmly mounted on one end of the rotary shaft 5 within the pump housing 4, and a mechanical seal 8 is mounted between the pump housing 4 and the rotary shaft 5 adjacent to the impeller 10. A flange 9 for mounting a pulley (not shown) is firmly mounted on the other end of the rotary shaft 5, to which a torque is transmitted from a crank shaft of the engine.
In this conventional water pump, known as a centrifugal type pump, the impeller is designed on the basis of the conventional design. In recent years, impellers made of steel plate as shown in FIGS. 12 and 13 or impellers made of plastic by injection moulding have been used with the view of reducing the manufacturing cost. Each of these impellers has thinner blades and therefore a wider passage at each of the blade inlets. Water pumps with such impellers are used at higher temperatures and with faster revolutions because they have superior anti-cavitation characteristics and have a longer service life as compared with the water pumps having conventional centrifugal impellers of cast iron. However, the water pumps with these new impellers have the disadvantages of greater noise and lower pump efficiency. Moreover, it is difficult to further improve their anti-cavitation characteristic due to an increase in circulation flow within the impeller.
Additionally, recently, with an increase in the output power of the engine, the quantity of heat radiating from the engine to the cooling water has been higher. In order to cope with this problem without effecting an increase in the cooling ability by increasing the size of the radiator or the cooling fan and so on, there is a demand for water pumps with a better cavitation characteristic. Furthermore, since the engine room is overcrowded by various equipment, it has been attempted to make the engine more compact, having a pump in any arbitrary position. This design also requires a water pump with a better anti-cavitation characteristics. In addition, since improvement of fuel consumption has also been required from the view of preventing air pollution, a water pump which is smaller in size, weight and higher in efficiency is also desirable. However, it would be impossible to effect larger improvements in pumps of conventional design because of the conventional design techniques. The operational point of a conventional water pump lies in the pump specific speed of 300 to 400 (m.rpm.M.sup.3 /min), which is the point at which the best pump characteristic is provided. This operational point makes the improvement of the impeller in the water pump difficult.